These little cars are so light, you would be wasting money on a power brake set-up. Just swap to disks and you will notice an improvement in braking power. I recommend Dennis Ginsberg's swap kit. It includes absolutely everything you need, and I believe he is still in the $600 neighborhood. It is an excellent kit at a decent price. http://www.discbrakeswap.com/
As far as the steering box is concerned they are a bitch to change to a new one like a flaming river box because there is no rag joint. You have to remove the entire steering column and replace the entire steering column. The first thing you should do is try to tighten the old one. Look at the top of the box, and you have a bolt sticking up with a notch that you can fit a screwdriver in. Turn your steering wheel all the way to the left, then loosen the nut and turn that notched bolt with a screwdriver until it bottoms out, then back off just a tad, then tighten the nut up.
As far as the suspension is concerned, look at the top of this Classic section and you will notice a FAQ section. Go there and look up "suspension," there is a ton of information there.